1. Field of the Invention
This invention relates generally to modem communications, and, more particularly, generating tones in a multi-tone modem.
2. Description of the Related Art
As the complexity of online content grows, many users find that typical connections, such as a 28.8 KBPS modem, are simply not fast enough. Even newer modems that allow communication speeds of 56.6 KBPS seem slow in some applications. A common bottleneck in online applications, such as the Internet, is telecommunications bandwidth. Projected demand for additional services, such as video-on-demand, teleconferencing, interactive television, and the like is likely to exacerbate the bandwidth problem.
One telecommunications protocol designed to alleviate the bandwidth problem is described in the asynchronous digital subscriber line (ADSL) standard, ANSI T1.413 Issue 2, entitled, "Interface Between Networks and Customer Installation--Asymmetric Digital Subscriber Line (ADSL) Metallic Interface, Rev. R6, dated Sep. 26 1997, incorporated herein by reference in its entirety.
ADSL modems use two competing modulation schemes: discrete multitone (DMT) and carrierless amplitude/phase modulation (CAP). DMT is the standard adopted by the American National Standards Institute.
The technology employed by DMT ADSL modems is termed discrete multi-tone. The standard defines 256 discrete tones. Each tone represents a carrier signal that can be modulated with a digital signal for transmitting data. The specific frequency for a given tone is 4.3125 KHz times the tone number. Tones 1-7 are reserved for voice band and guard band (i.e., tone 1 is the voice band and tones 2-7 are guard bands). Data is not transmitted near the voice band to allow for simultaneous voice and data transmission on a single line. The guard band helps isolate the voice band from the ADSL data bands. Typically, a splitter may be used to isolate any voice band signal from the data tones. Tones 8-32 are used to transmit data upstream (i.e., from the user), and tones 33-256 are used to transmit data downstream (i.e., to the user). Alternatively, all the data tones 8-256 may be used for downstream data, and upstream data present on tones 8-32 would be detected using echo cancellation. Because more tones are used for downstream communication than for upstream communication, the transfer is said to be asymmetric.
Through a training procedure, the modems on both sides of the connection sense and analyze which tones are less affected by impairments in the telephone line. Each tone that is accepted is used to carry information. Accordingly, the maximum capacity is set by the quality of the telephone connection. The maximum data rate defined by the ADSL specification, assuming all tones are used, is about 8 MBPS downstream and about 640 KBPS upstream.
The DMT ADSL modem described above is hereinafter referred to as "full-rate" modem because it utilizes all 256 tones (sometimes referred to as "sub-channels") as defined by the (ANSI) ADSL standard. The standardized DMT system uses the 256 tones in the forward (downstream) direction, where the downstream direction is typically construed as transmissions from a central office (typically owned by the telephone company) to an end-user. FIG. 1 illustrates the bandwidth requirements for a "full-rate" ADSL modem. To support the frequency bandwidth shown in FIG. 1, the "full-rate" ADSL modem must have an analog sampling rate of at least 2.2E+6 samples per second.
The DMT ADSL modem generally utilizes a 512-point Inverse Fast Fourier Transform (IFFT) for modulation and a 512-point Fast Fourier Transform (FFT) for demodulation. When transmitting a signal, the DMT ADSL modem uses a 512-point IFFT to generate an output signal from 256 complex data values. Those skilled in the art will appreciate that any two symmetrically located FFT bins form a Quadrature Amplitude Modulation (QAM) channel. The QAM channel can be modulated by a complex symbol, which means that it is possible to modulate the amplitude and phase of the QAM channel. All QAM channels except the N/2-th (e.g., 512.div.2=256.sup.th) channel can be modulated by a complex symbol, where N is the number of bins or channels of an IFFT unit. The N/2-th channel, which is typically not used by DMT ADSL modems, has a quadrature carrier identical to zero, and thus can be modulated only by a real symbol.
The DMT standard for a full-rate modem requires the generation of several necessary tones for communication. For example, the DMT standard requires generation of a 276 KHz pilot tone as well as a 310.5 KHz tone. The 276 KHz pilot tone is utilized for synchronization between two communicating modems. Because the specific frequency for a given tone is 4.3125 KHz times the tone number, the pilot tone (i.e., 276 KHz) corresponds to the 64.sup.th tone and the 310.5 KHz tone corresponds to the 72.sup.nd tone. The pilot tone is a complex tone, having a magnitude and a phase. The phase of the pilot tone, which is specified as 45 degrees, for example, can be utilized by a receiving DMT ADSL modem to synchronize an incoming signal.
The above described full-rate DMT ADSL modem has a high bandwidth for transmitting data, but is inflexible. All potential installations might not require the same bandwidth. In addition, due to the algorithm processing requirements, storage requirements, power consumption, gate count, analog sample converter rate, and physical size required to support the bandwidth shown in FIG. 1, the modem is costly, which may preclude its use for certain applications. Accordingly, DMT ADSL modems that support smaller bandwidth than full-rate modems are emerging in the market.
A quarter-rate DMT ADSL modem, for example, has a lower bandwidth than the full-rate modem but still uses the DMT scheme to communicate with other modems. A quarter-rate modem has a bandwidth of 276 KHz (1.104 MHz.div.4), and it operates at a sampling rate of 552KHz.
While utilizing lower bandwidth DMT modems has several advantages, there is, however, at least one shortcoming. Lower bandwidth DMT modems sometimes may not be compatible with full-rate DMT modems because of their inability to generate certain tones required by the DMT standard. That is, the lower bandwidth modems may not be capable of generating several necessary tones, such as the 276 KHz pilot tone and the 310.5 KHz tone, which are required by the DMT standard for a full-rate modem.
The 276 KHz and 310.5 KHz tones may not be readily generated in certain lower bandwidth DMT modems for a variety of reasons. For example, in a quarter-rate modem, the FFT has 128 bins (e.g., N=128), which means that channel 64 corresponds to the N/2-th channel. The 64.sup.th channel corresponds to the 276 KHz pilot tone that is required by a full-rate DMT modem and is typically modulated with a known amplitude and phase. However, because the 64.sup.th channel is the N/2 channel in a quarter-rate modem, it has a quadrature carrier that is equal to zero and cannot be modulated by a complex symbol (e.g., no imaginary component), as is required by the DMT standard for full modems. The 310.5 KHz tone, which corresponds to the 72.sup.nd channel, is simply not available in a quarter-rate modem that has a bandwidth of only 276 KHz (i.e., only 64 channels).
The present invention is directed to overcoming, or at least reducing the effects of, one or more of the problems set forth above.